The Hadith in question reads:
Sunan Abu Dawud 3991—Abu Dharr said: I was sitting behind the Apostle of Allah who was riding a donkey while the sun was setting. He asked: Do you know where this sets? I replied: Allah and his Apostle know best. He said: It sets in a spring of warm water.
This means, of course, that Muslims aren't allowed to reinterpret the Qur'an's obviously false claim that Dhul-Qarnain reached the place where the sun sets, and found the sun going down into a pool of muddy water:
Qur’an 18:83-86—And they ask you about Dhul-Qarnain. Say: “I shall recite to you something of his story.” Verily, We established him in the earth, and We gave him the means of everything. So he followed a way. Until, when he reached the setting place of the sun, he found it setting in a spring of black muddy (or hot) water. And he found near it a people.
For a full discussion of these passages, see my video:
See also Sam Shamoun's article, "Muhammad and the Sun’s Setting Place."